There have been conventionally known various types of substrate processing systems configured to perform a liquid process using a process liquid, a gas process using a process gas, or processes such as a cleaning process and a thermal process, to substrates such as semiconductor wafers. Such a substrate processing system is equipped with a plurality of substrate processing units for processing substrates. In addition, the substrate processing system is provided with a substrate accommodating part for temporarily accommodating substrates which are not yet processed by the substrate processing units, and substrates which have been processed by the substrate processing units. With the use of a substrate transferring apparatus, the substrates which are not yet processed by the substrate processing units are transferred from the substrate accommodating part to the substrate processing units, and the substrates which have been processed by the substrate processing units are transferred from the substrate processing units to the substrate accommodating part.
To be specific, for example, JP2003-273058A discloses a substrate processing system including a plurality of (specifically four) substrate processing units for cleaning substrates such as semiconductor wafers. In the substrate processing system disclosed in JP2003-273058A, the substrate processing units are disposed in two tiers in an up and down direction. One substrate transferring apparatus alternately transfers substrates from a substrate accommodating part to the respective substrate processing units disposed in two tiers in the up and down direction, and alternately transfers substrates from the respective substrate processing units disposed in two tiers in the up and down direction to the substrate accommodating part.